The Humminbird 581i HD DI Combo, is also one of the new models with HD display and Down Imaging, as it’s name suggests it. Although it only features a black and white screen, 16 lvl grayscale, the images for both SwitchFire and Down Imaging, and also the mapping displays, are quite clear and accurate.
The 581i HD DI Combo has a medium size screen, 5-inch diagonal, 640×320 pixel resolution. Along with the Down Imaging option, come quite a few other very useful features. For example, this unit is equipped with Precision GPS, chartplotting and trackplotting, having the UniMap mapping package built-in. It’s also compatible with the Navionics mapping package.
It can store in its memory chip up to 2,500 waypoints, 50 routes and 50 tracks (20,000 points each).
This unit has DualBeam Plus sonar, which means you can see both the sonar returns in split screen, and also the DI returns. The Down Imaging sonar beams can be operated at two frequencies: 455 kHz launching a conical beam of 75° or 800 kHz, with a beam of 45°. So 800 kHz for a sharper but narrower view, and 455 kHz for a less sharp image, but covering a wider area.
The transducer also uses conical beams to provide data in traditional 2D format. That would be a beam of 455 kHz – 16° cone center beam, or 200 kHz for a wider 25° beam.
All in all, the 581i HD DI Combo is reliable for waters up to 600 feet deep.